This invention concerns polymeric films and more particularly polymeric films having voided core layers.
Polymeric films having voided core layers typically consist of a core layer of a voided propylene homopolymer with one or more non-voided layers on each surface of the core layer. Such films have found wide acceptance as packaging materials and as label stock, for example for self-adhesive labels and labels which are to be applied to articles by methods such as in-mold labelling.
In general, voiding of the core layer of such films makes them opaque, which in itself can be a desirable property for many end uses such as labelling, and it increases the yield of film from a given amount of propylene homopolymer since a significant proportion of the core layer consists of vacuoles which are formed during the voiding process. Significant reductions in film density can therefore be achieved compared with that calculated from the materials used to form the films.
As with other polymeric films, end uses for polymeric films having voided core layers often include printing of one of their outer surfaces. However, it has been found that doing so can result in curling of the film. Whilst this may not lead to problems with films which are to be used for packaging by so-called form-fill-sealing, it can lead to serious problems with films which are to be used for labelling. This can be especially so with films which are to be used for in-mold labelling where individual labels are held within a mold and against a surface of the mold whilst an article is formed within the mold and the label is thereby incorporated into the surface of the article. Curling leads to label fold and/or the label failing to be correctly incorporated into the articles, e.g. containers lids and other surfaces.
According to the present invention there are provided biaxially oriented polymeric films comprising a core layer of a voided propylene homopolymer having a density of not more than 0.70 g/cm3, and at least one substantially non-voided layer on each surface of the core layer, the ratio of the combined thicknesses of the non-voided layers on the respective surfaces of the core layer being from 2:1 to 1:1.